1. Field of the Invention
The present invention relates to a switching power supply unit and an electronic apparatus including such a switching power supply unit.
2. Description of the Related Art
In printers, facsimile machines, and other electronic apparatuses, reduction of power consumption in a standby mode, that is, when no printing operation is performed, is in great demand. One demand is that, when a power supply unit for use in a printer or a facsimile machine is in standby mode, that is, in a light load mode, power consumption must be reduced.
It is impossible for a conventional RCC switching power supply unit to reduce its power consumption because it includes a feature in which, as load is reduced, a switching frequency increases, thus increasing switching loss.
An RCC switching power supply unit for reducing power consumption in a light load mode has been disclosed in Japanese Unexamined Patent Application Publication No. 7-67335. The switching power supply unit in Japanese Unexamined Patent Application Publication No. 7-67335 includes a circuit that grounds the control terminal of a first switch element for a desired amount time, whereby the turn-on operation of the first switch element is delayed such that power consumption in the light load mode is reduced.
Nevertheless, the switching power supply unit in Japanese Unexamined Patent Application Publication No. 7-67335 has a problem in that, by controlling only the switching frequency such that it is less than a desired level, substantial reduction in the power consumption by greatly lowering the switching frequency in the light load mode cannot be achieved.
Another problem is that the switching frequency cannot adjust to a rapid change of the load. For example, where settings are used such that the switching frequency greatly changes between the light load mode and a heavy load mode, when the load rapidly changes from light to heavy, the switching frequency cannot adjust to the change of the load, such that a decrease in the output and termination of power supply occurs. Accordingly, there is a problem in that the switching frequency cannot be substantially reduced in the light load mode.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a switching power supply unit that greatly reduces power consumption by greatly lowering a switching frequency in a light load mode, and an electronic apparatus including such a novel switch power supply unit.
According to a preferred embodiment of the present invention, a switching power supply unit is includes a transformer having a primary winding, a secondary winding, and a feedback winding, a first switch element connected in series to the primary winding, a control circuit provided between the feedback winding and a control terminal of the first switch element, a rectification circuit connected to the secondary winding, and an output-voltage detecting circuit which detects an output voltage output from the rectification circuit and feeds back the voltage to the control circuit. The control circuit includes a turn-off circuit for turning off the first switch element when it has been in an ON state, and an off-period control circuit which controls the off-period of the first switch element to be longer as a load is lighter, based on a feedback signal from the output-voltage detecting circuit, by delaying the turning-on of the first switch element.
Preferably, the off-period control circuit includes a second switch element which is provided between the feedback winding and the control terminal of the first switch element and which is controlled to be turned on and off based on the feedback signal from the output-voltage detecting circuit.
The second switch element may be an NPN transistor or an n-channel FET.
The switching power supply unit preferably includes a limit circuit for preventing a control voltage for the first switch element from exceeding a desired range, which is included in the second switch element.
The off-period control circuit preferably includes a second switch element provided between the feedback winding and the control terminal of the first switch element, and a time constant circuit which is arranged to the control terminal of the second switch element and in which a time constant is controlled based on the feedback signal of the output-voltage detecting circuit. The output-voltage detecting circuit further includes a negative feedback circuit for maintaining the feedback signal to be substantially constant.
The turn-off circuit also preferably includes an on-period extension circuit which makes the on-period of the first switch element longer as a period in which the turning-on of the first switch element is delayed by the off-period control circuit is shorter.
The turn-off circuit further preferably includes a third switch element which turns on the first switch element by connecting to the control terminal of the first switch element to be turned on. The on-period extension circuit further preferably includes a capacitor which is connected to the control terminal of the third switch element and which is controlled to discharge in accordance with the length of the off-period of the first switch element after charging in a reverse direction in the off-period of the first switch element, and is controlled to charge in a forward direction to a voltage turning on said third switch element during the on-period of the first switch element.
The off-period control circuit and the turn-off circuit included in the control circuit accelerate, based on the feedback signal from the output-voltage detecting circuit, the timing with which the turn-off circuit performs turning off during the off-period of said first switch element.
The off-period control circuit also preferably includes a phototransistor which receives the feedback signal from the output-voltage detecting circuit, and the emitter of the phototransistor is be connected to an on-period extension circuit such that a current flowing in the phototransistor charges the capacitor of the on-period extension circuit.
The control circuit further includes a voltage supply circuit for supplying a control voltage to the off-period control circuit by using a voltage generated in the feedback winding.
The switching power supply unit also preferably includes a direct-current voltage supply for supplying a driving voltage to the off-period control circuit by using the voltage generated in the feedback winding, and a constant voltage regulator provided between an input power supply and the output of the direct-current voltage supply that prevents a current from flowing in reverse.
According to another preferred embodiment of the present invention, an electronic apparatus includes at least one of the switching power supply unit according to the preferred embodiments described above.
According to a switching power supply unit of preferred embodiments of the present invention, a control circuit includes a turn-off circuit for turning off a first switch element, which has been in an ON state, and an off-period control circuit for, based on a feedback signal from an output-voltage detecting circuit, controlling the turning-on of the first switch element to be delayed longer as a load is lighter, and the off-period control circuit includes a second switch element which is electrically connected in series between a feedback winding and the control terminal of the first switch element and which is controlled to be turned on and off based on the feedback signal from the output-voltage detecting circuit, such that a switching frequency is set to be lower as the load is lighter to greatly reduce power consumption at light loads.
In addition, according to an electronic apparatus of a preferred embodiment of the present invention, by using a switching power supply unit of other preferred embodiments of the present invention, efficiency is greatly increased.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.